MIS devices frequently are used in the deep depletion mode, and it is important to characterize device behavior in this mode. It is well-known that the minority carrier generation rate and the minority carrier lifetime in the deep depletion region are important parameters in determining such device behavior. For example, the rate of minority carrier generation in the deep depletion region can influence transfer efficiency in charge coupled devices and refresh time in dynamic random access memories.
In the past, methods which determined the minority carrier generation rate and the associated generation lifetime took advantage of the fact that increasing the voltage applied to the gate electrode typically causes the capacitance of the device to decrease and causes the width of the depletion region to increase, and that under thermal equilibrium conditions at the threshold voltage, the capacitance reaches a minimum equilibrium capacitance, and the width of the depletion region reaches a maximum equilibrium width which remains relatively constant despite further increases in the applied voltage. Under non-equilibrium conditions and with an applied voltage greater than the threshold voltage, the capacitance of the device may fall below the minimum equilibrium capacitance, and the depletion region may expand beyond its maximum equilibrium width to form a deep depletion region.
For example, one method comprises the steps of measuring the capacitance across an MIS device as a voltage sweep of increasing magnitude drives it into a deep depletion mode and a voltage sweep of decreasing magnitude drives it back out of the deep depletion mode; the slope of a voltage versus capacitance curve for the forward sweep together with the difference in the voltages at the minimum equilibrium capacitance for the forward and reverse sweeps are determined and used to calculate the minority carrier generation lifetime in the deep depletion region.
In another method, a voltage pulse is applied to an MIS device to rapidly drive the device into the deep depletion mode in which its capacitance is below the minimum equilibrium capacitance. A fixed gage voltage is then applied to the device, and the time, known as the relaxation time, necessary for the generation of minority carriers to cause the capacitance to increase to the minimum equilibrium capacitance is then measured to determine the rate of minority carrier generation and the associated generation lifetime in the deep depletion region.
While these earlier methods of measuring the minority carrier generation rate generally have been satisfactory, there have been shortcomings with their use. For example, many of the earlier methods proved to be relatively inaccurate because they incorrectly assumed a uniform doping concentration in the deep depletion region. Additionally, methods involving the measurement of the relaxation time required a relatively long measurement time and complicated data treatment.
Accordingly, there has existed a need for an improved method for accurately measuring the minority carrier generation rate in the deep depletion region of an MIS device. The present invention meets this need.